1. Field of the Invention
The present invention relates to new methods for manufacturing electronic devices, particularly printed circuit boards and other packaging devices. Methods of the invention include sweller and etch treatments that produce printed circuit boards and other devices that are substantially more robust and reliable than those produced by prior methods.
2. Background Art
Double-sided and multilayer printed circuit boards are used for a variety of applications and provide notable advantages of conservation of weight and space. A multilayer board is comprised of two or more circuit layers, each circuit layer separated from another by one or more layers of dielectric material, which generally is a resin/prepreg composite. Circuit layers are formed from copper clad laminate. Printed circuits are then formed on the copper layers by techniques well known in the art, for example print and etch to define and produce the circuit traces. As referred to herein, the term "multilayer printed circuit board" is inclusive of both double-sided boards and boards with greater than two circuit layers.
One traditional method for fabricating multilayer boards comprises fabrication of separate innerlayers having circuit patterns disposed over their surface. A photosensitive material is coated over the copper surfaces of a copper clad innerlayer material, imaged, developed and etched to form a conductor pattern in the copper cladding protected by the photosensitive coating. After etching, the photosensitive coating is stripped from the copper leaving the circuit pattern on the surface of the base material. Following formation of the innerlayers, a multilayer stack is formed by preparing a lay up of the innerlayers, ground plane layers, power plane layers, etc., typically separated from each other by a dielectric prepeg (a layer containing glass cloth impregnated with partially cured material, typically a B-stage epoxy resin). The stack is laminated to fully cure the B-stage resin.
Interconnections or through-holes, buried vias and blind hole interconnections (in general, "aperatures") are used to electrically interconnect circuit layers within a multilayer board. Buried vias are plated through holes connecting two sides of an innerlayer. Blind vias typically pass through one surface of the stack and pass into and stop within the stack. Regardless of the form of the interconnection, holes are generally drilled at appropriate locations through the stack, catalyzed by contact with a plating catalyst and metallized to provide electrical connection between the layers. This manufacturing process can pose some notable problems such as drill wear, smear and hole size limitations, all inherent in such a drilled board approach.
A more recent approach to multilayer circuit board manufacture that can avoid such drilled board problems includes sequential formation of board layers using plating techniques and imaging or laser ablation of dielectric materials to achieve fine line resolution and interconnections between circuits. Such a manufacturing process has been disclosed in U.S. Pat. No. 4,902,610.
In the manufacture of circuit boards and other electronic packaging devices, surface roughening and resin desmear of through holes and other apertures are performed chemically prior to metal plating of same to provide electrical interconnection. That roughening and desmear chemical treatment enhances the bond strength and electrical performance of the metal deposited on the aperture surface. That chemical treatment also removes resin smear such as from the board inner layer, enhancing metal-on-metal bonding. Resin smear is recognized herein to include any residual resin on copper or innerlayer of a circuit board or other device, such as may occur from drilling, incomplete development from photolithography, residues from laser ablation, detc. The roughening and desmear treatments is often referred to as "swell and etch" chemistry or treatment. The sweller formulation penetrates into and softens the resin of the circuit board or other device. The subsequently applied resin etch then oxidizes and in some cases can dissolve the polymer.